The present invention relates to a charging/discharging circuit with fast operation, especially to a charging/discharging circuit using a negative reference voltage to speed up operation.
The conventional charging/discharging circuit can be implemented by semiconductor switching elements. For example, the U.S. Pat. No. 4,754,175 xe2x80x9cSolid state relay having a thyristor discharge circuitxe2x80x9d by Kobayashi et al. discloses a charging/discharging circuit composed of a plurality of diodes and resistor. The resistor with larger resistance can suppress leakage current during charging operation. However, the response time of discharging operation is degraded due to reduction of discharging current. On the contrary, the resistor with smaller resistance can accelerate discharging operation, while the charging operation is degraded due to large leakage current. Therefore, the resistance is a trade off factor in charging/discharging circuit design.
Moreover, the charging/discharging circuit disclosed in above-mentioned patent comprises undue number of electronic elements; the process thereof is cumbersome. The conventional charging/discharging circuit generally uses ground level as lowest reference level for discharging operation. The response time of the discharging operation thereof is not satisfactory.
As shown in FIG. 1, another prior art charging/discharging circuit is the U.S. Pat. No. 4,931,656 xe2x80x9cMeans to dynamically discharge a capacitively charged electrical devicexe2x80x9d by Ehalt et al. This patent discloses a charging/discharging circuit composed of a resistor 40, an NPN transistor 44 and a photodiode 42. The photodiode 42 is used as switch element of the NPN transistor 44 for charging/discharging operation.
When the LED array 20 is driven to emit light, the photodiode array 30 receives the light and conducts a charging current flowing to an energy-storing element (a FET shown in this figure) 50 along a path C1 indicated by a dashed arrow shown at top of this figure. Most of the charging current then flows to the energy-storing element 50 along a path C2 due to the isolation provided by the resistive circuit 40. At the same time, the photodiode 42 also generates a photo current in response to the light of the LED array 20. The photo current of the photodiode 42 flows the emitter E of the NPN transistor 44 along a path C3. The NPN transistor 44 is cut off whereby the charging current is not leaked through the NPN transistor 44 and can rapidly charge the energy-storing element 50.
During the discharging stage of this charging/discharging circuit, the photodiode 42 does not receive light sufficient to conduct a photo current, part of the discharging current from the energy-storing element 50 flows to the base B of the NPN transistor 44 through a resistor 46 and along the path D3. Therefore, the NPN transistor 44 is turned on or saturated to conduct most of discharging current along paths D1 and D2.
However, in above-mentioned charging/discharging circuit, a resistive element 46 is still present, which imposes a dilemma to circuit design. Moreover, the above-mentioned charging/discharging circuit also uses ground level as lowest reference level for discharging operation. The response time of the discharging operation thereof is not satisfactory.
It is the object of the present invention to provide a charging/discharging circuit with fast operation
In one aspect of the present invention, the charging/discharging circuit has an energy-storing element such as a capacitor on a discharging path thereof. During the charging stage, the capacitor is charged by a photodiode and establishes a negative reference voltage. The negative reference voltage can speed up discharging operation.
In one aspect of the present invention, the charging/discharging circuit uses transistors and diodes to replace resistive elements and the drawbacks caused by the resistive elements can be prevented.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: